1. Field of the Invention
This invention relates to a process of creating polymeric foams from 0-100% recycled polymers, the foam created thereby retaining its original chemical structure and chemistry so that it may readily be recycled again using existing recycling processes to recover solid polymer which may in turn be foamed again. More specifically, this invention relates to the creating of foam or cellular polymers of density ranges of 3 to 99% of solid using any combination of virgin polymer, recycled pre- and post-consumer solid polymer, and recycled foam polymer that has been foamed and optionally thermoformed by the method of this invention including trim and other scrap material from the process of manufacturing foamed polymer articles.
2. Description of the Related Art
Articles of foamed polymeric material are ubiquitous in modern life. In industries ranging from food service and distribution, to packaging, to construction, modern society demands a plethora of items made of foamed polymer. Heretofore, however, the general manufacturing processes used to meet the demand for such items have inherent inefficiencies and high environmental costs.
A substantial volume of foamed polymeric articles are fabricated of solid state microcellular polymeric foam. Typically, such foams have a bubble density of more than 108 cells per cm3 with bubble diameters on the order of 10 μm. Compared with conventional solid polymers, solid state foamed polymers offer the possibility of a 3-99% reduction in material used while maintaining the essential mechanical properties of the unfoamed polymer. This in turn offers significant savings in material and transportation costs. Furthermore, foamed polymers are good insulators in general, so that materials of microcellular polymeric foam may be advantageously employed as thermal insulators in building construction and, indeed, are preferred materials for articles such as containers for hot and cold food and beverages.
The large amount of plastics in landfills is a widely recognized problem. Volume, degradability and hazards of waste are primary concerns regarding the waste stream. While efforts have been made to reduce the volume of waste, concerns continue regarding the accumulation of non-decomposable waste in the form of plastics and plastic foams. Effective, comprehensive recycling of pre- and post-consumer waste provides solutions to these concerns.
Heretofore, however, waste foamed polymeric material and articles (whether thermoformed or not) have been extraordinarily difficult to recycle. The prior art processes of foaming and forming polymer bring about irreversible changes in its constituents such that uses for the resulting waste foamed polymer are greatly exceeded by the amount of waste produced. In recent times, the majority of manufacturing scrap and pre- and post-consumer waste foamed polymer has simply been discarded, occupying our landfills.
Prior art foamed thermoformed polymeric articles have typically been manufactured in a process involving two stages: foaming extrusion and thermoforming. Foaming extrusion, as discussed in more detail below, entails producing or forcing a non-reactive gas into a molten polymer mixture, thereby forming bubbles in the melt. The mixture is allowed to cool and harden around the bubbles, which become small, gas-filled cells in the now solid foam material. Thermoforming, as will also be discussed in greater detail below, entails heating solid foam material until it is soft and pliable and then molding it into the shape of articles which the foam assumes in rigid form upon cooling.
As is well known to those in the art, for the prior art production of foamed articles of acceptable quality, the molten polymer mixture that is extruded must be of a certain minimum viscosity and melt hardness. In their pure, unaltered forms, the viscosity and melt hardness of the principal polymers used in the manufacture of thermoformed foam articles are inadequate for such purpose, particularly so in the case of polyesters such as polyethylene terephthalate (PET) and polyethylene (PE). Accordingly, in the prior art, various treatments of pure polymer resin are employed to confer proper viscosity to the mixture for foaming extrusion.
One treatment commonly employed to enhance polymer viscosity is simply to add viscosity enhancing additives to the resin mixture as it is melted. In the case of PET, such additives include very high molecular weight polyesters or branched polyesters, non-crystalline copolyesters, branching agents as taught in U.S. Pat. No. 5,288,764 to Rotter et al., and high molecular weight vinyl aromatic modifiers as taught in U.S. Pat. No. 5,310,799 to Carson et al. As is well known to those of skill in the art, the precise composition and proportion of additive, varying greatly with the polymer to be foamed and the requirements of the specific foaming extrusion process under consideration, is generally determined empirically on a case by case basis by trial and error.
Another treatment of polymer resins commonly employed in the production of prior art foamed polymers is cross-linking, whereby the polymer molecules in the polymer mixture are caused to be partially cross-linked with one another. Cross-linking is used to enhance polymer melt viscosity to enable or facilitate foam extrusion for many polymers, such as polystyrene and PET. In the case of polyethylene, cross linking of the polymer resin is used to increase the allowable service temperature of the polymer foam for such applications as under-hood automotive and steel roofing insulation. Cross-linking is usually induced by causing the formation of free-radicals in the polymeric material which then cause cross-linking between polymer molecules.
Most commonly, cross-linking of polymers by free radical reaction is accomplished by one of three methods: exposing melt-extruded sheet polymeric material to ionizing radiation; melt-mulling peroxide into the thermoplastic resin at a temperature below the decomposition point of the peroxide then heating the mixture to a temperature above the decomposition point of the peroxide; or melt-mulling the resin with an organo-functional silane. For some polymers, such as polypropylene homopolymers and copolymers, efficient cross-linking by free radical reaction requires the further addition of a cross-linking promoter or sensitizer such as: multifunctional vinyl monomers and polymers; divinylbenzene; acrylates and methacrylates of polyols; allyl alcohol derivatives; polybutadiene; and co-polymers of α-olefins and a non-conjugated diene.
In general, successful cross-linking by free radical creation and reaction requires that the free radicals produced principally cause formation of cross-linked polymers and only minimally cause scission of polymer molecules. For applications in which free radical formation results in substantial chain scission in polymer molecules, a multi-functional azide is instead employed to bring about cross-linking through a nitrene insertion reaction.
It is also often desirable to increase the rate at which semi-crystalline polymers such as PET crystallize when they are heated to temperatures below their melting point. By doing so, it is possible use semi-crystalline polymers economically to manufacture articles whose utility depends upon qualities possessed by the polymer only when it has a high degree of crystallinity. For example, foamed articles made of highly crystallized PET may be manufactured that are suitable for use in high service or operating temperatures at which less crystallized foamed PET articles are unsuitable. However, in the prior art it is not economical to form such crystallized foamed PET articles starting with standard foamed PET because of its relatively slow rate of crystallization.
A group of additives is used in the melt mixture in the prior art to enhance the crystallization rate of such semi-crystalline polymers. In the case of PET, polyolefins are added to the melt mixture to form a grade of PET, known as CPET, that is used to make high service temperature objects. The polyolefins serve as a nucleating agent to enhance crystallization of formed, foamed articles of CPET. The current industry practice for producing high service temperature objects is to form solid sheet (unfoamed) CPET and then raise the crystallinity level after forming using additional heat. Without the additives, the crystallinity level of the PET rises too slowly with applied heat for economic production of highly crystallized articles suitable for high service temperatures.
In any case, as is clear from the foregoing, and as is well known to those of skill in the art, virgin polymeric resin must be considerably chemically or molecularly modified, generally either by cross-linking molecules in the polymer or by processes involving melting the polymer with the addition of other compounds, before it can be successfully extruded as foam in the prior art. It is also clear that, in the prior art, solid or foamed PET can not economically be turned into high service temperature objects without additives and that polyethylene foam can not serve in high temperatures without cross linking.
Turning to examine the prior art foam extrusion process in more detail, extrusion involves foaming melted polymer with gas. Gas may be produced chemically by additives within the melted polymer, or it may be physically introduced into the melt by blowing. In some applications, both chemical production and blowing of gas are employed for foaming.
For chemical foaming, a pelletized resin of a polymer, such as polystyrene or polyethylene terephthalate (PET), treated for extrusion as described above, is fed into an extruder with blowing agent, such as p,p′-oxybis(benzenesulfonyl hydrazide), azodicarbamide, alkaline earth metal carbonates or bicarbonates such as calcium carbonate, magnesium carbonate or sodium bicarbonate, and combinations of an alkaline earth metal carbonate or bicarbonate and one or more organic acids such as citric acid.
In addition, the treated resin mixture may be further mixed with optional polymer scrap, the relative proportion of which, significantly, must be limited, as will be discussed in greater detail below.
In the extruder, the resin mixture is heated above the polymer's glass liquefaction or melt temperature (about 265 deg. C. for PET and about 240 deg. C. for polystyrene). If physical blowing is used to cause foaming, resultant melted mixture is then blown with carbon dioxide, or a hydrocarbon gas such as pentane, or an HFC (fluorocarbon) gas to produce foam. On exiting the extruder, the foamed sheet may have uneven rounded edges. On cooling and solidifying, the foamed material is typically cut and trimmed into sheets which are wound into rolls. The rolls of foamed polymer are then generally taken to an outside storage area for several days of curing prior to further processing.
The curing process is generally necessary, for the following reasons. Because of thermal contraction of the foaming gas, the now rigid cells in the newly cooled foamed polymer contain the foaming gas at a pressure considerably less than atmospheric pressure, on the order of 0.5 atmospheres. Because the cell walls are more permeable to atmospheric gases than to the various gasses used in foaming gas, during curing the atmospheric gases osmotically penetrate the cells in the foamed polymer, actually increasing the pressure in the cells to above atmospheric pressure, on the order of 1.5 atmospheres. The additional pressure in the foamed polymer cells facilitates thermoforming and in fact results in secondary expansion of the polymer material during the thermoforming stage.
During curing also, however, trapped foaming gas escapes from the foamed polymer into the atmosphere. When residual foaming gas is undesirable in the finished product, as may often apply to polymers foamed with hydrocarbon gas, it has been desirable with the prior art to allow the escape of trapped foaming gas. However, the gases released during curing in the prior art contribute to environmental pollution. Hydrocarbon gas release contributes to low altitude smog, while HFC fluorocarbon gas release is known to have an ozone-destructive effect.
In the stage of thermoforming articles of foamed polymer, sheets of foamed polymer are fed through an oven to heat the polymer close to its softening point. The hot polymer sheet is then forced into molds by vacuum, air or mechanical pressure. As is well known to those of skill in the art, for prior art untreated foamed polymer in general, if open molding is used the foamed material on the open side of the mold is unacceptably porous and uneven for most applications, and so closed molding is generally used. On cooling to the point where it is again somewhat rigid, the molded foam sheet is fed through a trim press where the desired articles are cut from the thermoformed sheet by a punch and die mechanism.
Both stages in the prior art process of producing thermoformed foamed articles result in the creation of scrap polymer. In the extrusion stage, scrap consists principally of excess foamed polymer from the extruder in foam sheet form and results from machine start-ups, and size and color changes as well as scrap from cutting and trimming rounded edges. In the thermoforming stage, scrap comprises principally trimmings (intrinsic to the thermoforming process) and to a lesser extent rejected defectively formed articles. While efficiencies in production have been employed to reduce the amount of scrap in the prior art process of manufacturing foamed polymer articles, typically only 60 to 75% of the polymer is fabricated into articles on a single pass, the remainder set aside as scrap.
For environmental reasons, it is highly desirable to recycle the scrap resulting from the fabrication of thermoformed formed articles. Ideally manufacturing whereby scrap from manufacture is simply refoamed and thermoformed in the normal production process, would address the need for recycling of foamed polymer scrap. However, heretofore such manufacture of foamed polymeric materials has been severely limited.
As discussed above, in the prior art some of the foamed polymer scrap from the manufacturing process may be reused to make new foam polymer, by inclusion of the scrap in the resin mixture to be melted and extruded. For example, U.S. Pat. No. 6,130,261 to Harfmann teaches processing scrap PET foam for reuse by flaking, pelletizing and then desiccating the scrap material.
In practice, however, there is a relatively low limit in the amount of scrap foam produced by the prior art that can be included in the resin mixture while still producing foamed polymer material and articles of acceptable quality. This is because the prior art finished foamed polymeric material differs not only physically but also chemically to a greater or lesser extent from the initial resin that is melted for foaming in the extruder, for several different reasons discussed in further detail below. The chemically different prior art foamed polymeric material in general is not suitable for inclusion in large proportions in the extrusion resin mixture, thereby limiting the availability of closed-loop manufacturing for such material. Accordingly, as a general rule, foam producing plants limit their reuse of self-generated scrap, if it can be used at all, to at most 20 to 50% by weight of the extrusion resin mixture.
Foamed polymeric material will contain the additives used to enhance the viscosity of the melted resin for extrusion. It may further contain contaminant in the form of residual blowing agent or products of the blowing reaction, such as metallic cations and/or conjugate bases of organic acids such as citrate, depending upon the extrusion process used to manufacture the material. Yet further, it may contain contaminant in the form of residual cross-linking additives and sensitizers. Remelt and extrusion of the material with added blowing agent and/or other contaminants leads to the accumulation of such contaminants with each recycling, thereby altering the composition of the recycled foamed polymer.
Perhaps more significant, as is known to those of skill in the art, when prior art foaming thermoplastic polymers are repeatedly broken up, melted and extruded, their chemical structure breaks down, with average molecular weight of the polymer declining in thermo-oxidative and mechanical degradation with each successive extrusion. While this degenerative effect of recycling is less pronounced for some polymers, such as PET, than it is for others, such as polystyrene, as a general rule the extrudate from a melted polymer foam has significantly lower melt viscosity than the extrudate from which the foam was originally formed. In the case of polystyrene foam, the reduction in melt viscosity from polymer degradation is so pronounced that acceptable extrusion foaming is simply not possible for melt mixtures with high percentages of the recycled foamed polymer. For many other polymers, when higher percentages of recycled foamed polymer are used in the melt mixture for prior art foamed polymers, successful extrusion, if indeed it is possible at all, usually requires the addition of viscosity enhancers to the melted reused foam.
For example, U.S. Pat. No. 5,391,582 to Muschiatti et al. teaches use of functionalized chain extenders and cross-linking agents to enhance the viscosity of recycled PET foam. Among such additives are: acid, epoxy and anhydride functionalized ethylene copolymers; partially neutralized ethylene methacrylic acid and acrylic acid copolymers; polyester thermoplastic elastomers; low molecular weight carboxylic acids, acid anhydrides, polyols, and epoxies. These and other viscosity-enhancing components can be incorporated up to about 20% by weight based on the weight of the recycle PET. When recycle PET is melt blended with such additives, the blended recycle may comprise up to 75% of a resin mixture suitable for extrusion, if the remaining 25% or more of the resin mixture made up of a branched PET component. While such additives may give the resin mixture sufficient viscosity for satisfactory extrusion, however, the resulting extruded foam is no more easily recycled than its progenitor.
Yet another common reason that prior art finished foamed thermoplastic material differs chemically from the polymer resin from which it is formed is that it is often desirable for any of a number of reasons for foamed polymeric articles to possess a relatively impermeable, smooth surface or skin. Such skin may act to improve rigidity, provide a gas barrier, resist abrasion, enhance appearance, enhance stain resistance, and impart other desirable qualities. To achieve such results, prior to or at the time of thermoforming the article, the surface of the foamed polymer sheet is often bonded to a smooth, non-permeable polymer film, typically vinylidene chloride copolymer, known as saran, to form so-called “barrier foam”, as taught in U.S. Pat. No. 4,847,148 to Schirmer.
It is found, however, that when barrier foam scrap containing saran is heated to the foam polymer melt temperature in the extruder, it burns and cannot be used for extrusion. To address the problem of recycling barrier foam scrap, film materials other than saran have been used. For example, U.S. Pat. No. 5,330,596 to Gusavage et al. teaches use of ethylene vinyl alcohol, acrylonitrile copolymers and/or nylon copolymers for barrier film. While scrap from such barrier foam may be included up to certain proportions in the extruder resin melt, it is nonetheless different in composition from the virgin foaming polymer resin and results in a somewhat different extrudate. Furthermore, an extruded foam polymer, even with a recyclable barrier layer, suffers from chemical degradation on subsequent remelt and extrusion, as discussed above.
It is further highly desirable to maximize the amount of post-consumer recycled material used as source polymer in the manufacture of foamed polymer, regardless of whether or not such material was previously foamed. Since, however, recycled material in general will have been manufactured for different items (from toys, packages, office machines, etc) using as many different processes and process parameters as there are manufacturing plants, the amount of chemical variation in the recycled polymer will be even greater than for the manufacturer's own scrap, presenting all the problems for the prior art processes discussed above. Furthermore for many products, the consumer's use will cause additional variations in the chemistry. For instance exposure to sun or high heat will break down molecules and storing chemicals or food temporarily may affect the polymer. These variations make it more difficult to use the prior art foaming process to foam a post-consumer recycled polymer than to foam plant scrap or pre-consumer recycled plastics.
What is needed is a polymer foam that may be easily rendered to be chemically identical to its unfoamed progenitor. Further, what is needed is a way of producing foamed polymer that does not result in chemical degradation of the polymer. Further required is a way of producing foamed polymer that limits thermal degradation of polymer during processing by using the lowest processing temperatures possible. Yet further, what is needed is a barrier skin for foamed polymeric materials that does not alter the chemistry of the polymer. Further still is required a process that is not limited by the relatively low melt viscosities of polymers that are commonly foamed and that does not result in lowering the melt viscosity of the polymeric material on foaming. Yet further, what is needed is a foaming process that does not employ environmentally detrimental gasses. For truly closed-loop manufacture of materials and articles of foamed polymer, what is needed is a process of foaming polymer that is suitable for foaming substantially all manufacturing scrap material produced in the process as well as recycled pre- and post-consumer plastics of widely varying composition, whether previously foamed or not.
It is known that microcellular foamed polymer may be produced by methods that differ significantly from the blown foaming extrusion method described above. Such foam may be produced in the solid state by a two-step batch process, such as described in U.S. Pat. No. 4,473,665 to Martini-Vvedensky, et al., in which a solid polymer is first exposed to a non-reacting gas, such as carbon dioxide or nitrogen, at elevated pressure for a period of time sufficient to achieve a concentration of gas in the polymer which is sufficient to permit bubble nucleation. After exposure to the gas the polymer is subsequently returned to normal pressure, producing a supersaturated sample, and heated to the foaming temperature, which is above the glass transition temperature of the gas-saturated polymer, thereby causing a large number of bubbles to nucleate in the polymer. The polymer is held at the foaming temperature for a period of time sufficient to achieve foam of the desired density and then cooled to quench bubble nucleation and growth. Because the process is a batch process, however, suitable for production only of single foamed polymer sheets, this technology is of limited applicability to most of foamed polymeric material fabrication, which requires scalable continuous or semi-continuous production of foamed polymeric material.
More recent discoveries have adapted such gas-impregnated foamed polymer technology to “semi-continuous” production processes. In U.S. Pat. No. 5,684,055 to Kumar et al., incorporated herein by reference in its entirety, a roll of polymer sheet is provided with a gas channeling means interleaved between the layers of polymer. The roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve a desired concentration of gas within the polymer. The saturated polymer sheet is then separated from the gas channeling means and bubble nucleation and growth is initiated by heating the polymer sheet. After foaming, bubble nucleation and growth is quenched by cooling the foamed polymer sheet.
In U.S. Pat. Nos. 5,223,545 and 5,182,307 Kumar et al., both is incorporated herein by reference in their entirety, PET is shown to have its crystallinity levels raised by saturation with high pressure CO2 gas. Furthermore it has been shown that the crystallizing gas remains in the polymer in substantial quantities after foaming.
It has now been discovered, surprisingly, that the foamed polymer produced by gas impregnation may be reduced to a form that is virtually identical in melt viscosity and average molecular weight to the polymeric resin from which it was formed. Another surprising discovery is that high pressure gas impregnation results in polymer that may be foamed at temperatures significantly lower than the nominal glass liquefaction temperature of non-saturated polymer. Further, successful foaming of gas impregnated polymer is independent of the polymer's melt viscosity and therefore is unaffected by variations in the polymer melt viscosity such as found in recycled material. The gas impregnated foaming process easily accommodates variation in polymer glass transition temperature typically found in recycled material by adjustment of foaming temperature. It has yet further been discovered that relatively impermeable smooth skins may be induced on the surface of the gas impregnated foamed polymeric materials that are comprised of crystallized forms of the polymer itself. It has also been discovered that semi-crystalline polymers, whose crystallinity has been raised by saturation with high pressure CO2 gas, and which are thermoformed while substantial gas remains in the polymer, have their crystallinity levels increased after forming without the need of nucleating additives by using plasticizing gasses as blowing agents. Based upon these discoveries, it is an object of this invention to use a wide range of polymeric materials, including up to 100% pre- and post-consumer recycled plastic and manufacturing scrap, whether of foamed structure using methods of this patent or of solid structure, to make foamed polymeric materials and articles by a closed-loop process of manufacturing by solid state foaming. This and other objects of the invention will be apparent to those skilled in this art from the following detailed description of preferred embodiments of the invention.